Summary

The different colors on the surface of a soap bubble arise from the interference of light waves reflecting from the outer and inner surface of the liquid film. As the thickness of the film varies, so will the wavelength of light that undergoes constructive interference and remains visible. This effect can be used to measure small changes in distance if a single coherent beam is used (see the figure, panel A) through the formation of interference fringes. According to quantum mechanics, even material particles such as electrons behave like waves, and indeed, interference can be observed when the electric charge associated with an electron travels along two arms of a ring-like interferometer in the Aharonov-Bohm effect. In addition to their charge, electrons also have two distinguishable spin states, spin-up and spin-down. On page 669 of this issue, Petta et al. (1) demonstrate beam splitting and interferometry for the spin degrees of freedom of two electrons on a semiconductor chip. In this system, the phase of partial waves is associated with spin direction. Nuclear spins, whose coupling to electrons can destroy phase coherence, actually help control spin-state evolution.